1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a configuration in which a sheet is separated and fed by blowing air onto the sheet.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine, a sheet feeding apparatus of an air feeding type using air has been discussed to feed, from a sheet storage unit in which sheets are stacked and stored, the sheets one at a time. The sheet feeding apparatus blows air onto upper edges of the stacked sheets to float and loosen a plurality of sheets, and then attracts only one of the sheets to a suction conveyance belt arranged in its upper part and conveys the sheet.
The sheet feeding apparatus of an air feeding type blows air from a loosening nozzle 33 onto front edges of sheets S stored in a sheet storage unit 100 to float a plurality of upper sheets to loosen the sheets, as illustrated in FIG. 6. The uppermost sheet is attracted to a suction conveyance belt 21 at negative pressure. At that time, the uppermost sheet is separated from the sheets S by air blown from a separation nozzle 34. The uppermost sheet that has been attracted to the suction conveyance belt 21 is conveyed when the suction conveyance belt 21 rotates, and is conveyed to a drawing roller pair 105 located downstream while being guided by a conveyance guide 15.
FIG. 7 is a side view of the sheet feeding apparatus illustrated in FIG. 6 as viewed from the left. The center of the floated sheet S is floated by air blown form the loosening nozzle 33. However, both edges of the sheet S in a sheet width direction perpendicular to a sheet conveyance direction cannot be floated but hang because air is not blown thereto. Particularly, a difference in height most significantly appears for an uppermost sheet Su that has been attracted to the suction conveyance belt 21 (indicated by B-P in FIG. 7). The larger the size of the sheet S becomes, or the lower the rigidity of the sheet S becomes, the larger the difference in height becomes. When the sheet S is conveyed in this state, the sheet S may be folded at its corner of the leading edge or jammed.
A technique for solving this issue is discussed in U.S. Patent Publication Application No. 2007/222138. The technique relates to a conveyance guide 15 having a shape as illustrated in FIG. 8. A guide surface of the conveyance guide 15 has, in a sheet conveyance direction indicated by an arrow A in FIG. 8, inclined portions 15A and 15B which are inclined from the upstream side to the downstream side, and formed therein from the center to both ends in a sheet width direction.
FIGS. 9A and 9B illustrate how an edge of a sheet S to be conveyed in a hanging state by the conveyance guide 15 is lifted as the sheet S is conveyed. More specifically, the edge of the sheet S, which has been attracted to a suction conveyance belt 21, hangs before it abuts on an end portion of the upstream side of the conveyance guide 15 in the sheet conveyance direction. When the suction conveyance belt 21 conveys the sheet S, the edge of the sheet S contacts a point 15A1 of the inclined portion 15A of the conveyance guide 15 (the sheet S is in a state Sa), and the sheet S is lifted along the inclined portion 15A.
As the sheet S is further conveyed, its contact point with the inclined portion 15A gradually moves to points 15A2 and 15A3 outside the sheet S, and the sheet S is changed to states Sb and Sc respectively. More specifically, the sheet S can be smoothly guided to a sheet conveyance unit located downstream while the inclined portion 15A of the conveyance guide 15 lifts a hanging portion of the sheet S. Therefore, even if the sheet S is in a hung state, the sheet can be satisfactorily conveyed without being folded at the corner or jammed.
A rate of occurrence of corner folding at the edge or jamming of a sheet can be significantly reduced by providing the above-described conventional conveyance guide. However, the above-described conventional conveyance guide cannot sufficiently handle coated paper having a grammage of less than 80 g/m2, non-coated paper having a grammage of less than 50 g/m2, a large-sized sheet having a length of more than 13 inches (approximately 330 mm) in a direction perpendicular to the conveyance direction, or a sheet having an edge previously curled. More specifically, the above-described conventional conveyance guide cannot sufficiently lift the sheet of these types since its edge too greatly hangs, so that the rate of occurrence of corner folding or jamming may be increased. Further, in a use environment where the sheet absorbs moisture and its edge greatly hangs, the rate of occurrence of corner folding or jamming is further increased.